This invention relates to the grinding of non-metallic hard materials, each having a Vickers hardness value up to 5000, and in particular to a method of grinding a non-planar surface on to a workpiece of such a material by employing a rotating grinding wheel.
The term "non-metallic" is employed in this specification and the accompanying claims to refer to any composition not comprising a metallic element, or an alloy of metallic elements, but possibly having at least some of the properties associated with a metallic element, or an alloy of metallic elements. Further, a surface on a workpiece of any such non-metallic composition is ground by the disintegration of the surface thereof, and the removal of small particles therefrom.
The invention relates particularly to a method of grinding a required non-planar surface on to a workpiece of a non-metallic hard material, in which method the axis of rotation of the grinding wheel used is caused to traverse along a predetermined axis relative to the surface of the workpiece blank so that the grinding wheel passes through a portion of the workpiece. It is known for the rotational axis either to reciprocate in the plane containing all the radii of the wheel (hereinafter called "the radially extending plane"), or to move only in one linear direction in this plane, relative to the workpiece surface, the predetermined axis lying in this plane. The wheel traverses relatively to the workpiece until the wheel has cut the full depth of its form into the workpiece. Hence, any undesired features in the required non-planar workpiece surface caused by imperfections of the grinding wheel extend linearly parallel to the predetermined axis, and to the radially extending plane of the grinding wheel.
It is also known that the required shape of the working surface of the grinding wheel is shaped by a tool with a complementary shape. The tool is required to pass through the wheel by traversing relatively to the wheel. The normal method of presentation is for the wheel to reciprocate relatively to the tool until the tool has cut the full depth of its form into the wheel. In particular, the present invention relates to a grinding method including such a method of shaping the working surface of the grinding wheel with a tool.
Usually the rotating grinding wheel has the appropriately shaped working surface, comprising the radially outer periphery of the wheel, formed from gemstone, or synthetic diamond, particles bonded to a suitable substrate.
A grinding wheel having a working surface of gemstone, or synthetic diamond, particles inevitably provides a surface on the workpiece with undesired grooves, having ridges therebetween. The ridges extend parallel to the direction of traverse of the axis of rotation of the wheel relative to the workpiece surface. This is because the particles have different sizes, and there is insufficient control over the way in which the particles are embedded in the working surface of the grinding wheel. The working surface is, thus, irregular with particles protruding therefrom by different amounts from what can be considered to be the general level of the working surface. The finish of such a working surface, conveniently, can be defined by the maximum amount of protrusion of the diamond particles from the general level of the working surface, such maximum particle protrusion being greater for a relatively coarsely finished working surface than for a relatively finely finished working surface.
It is known that, if the required workpiece surface is to be as flat as possible, the height of the ridges may, in the case of a planar surface, be at least reduced by cross-grinding in a direction at right angles to the direction of the initial grinding action. However, in grinding a non-planar workpiece surface such cross-grinding cannot be employed.
It is an object of the present invention to provide novel and advantageous method of grinding, in which method the height of ridges normally inevitably formed on the workpiece surface, can at least be reduced.